Matrix-type display apparatuses with a memory effect include not only phase transition liquid crystal display apparatuses as disclosed in Japanese Unexamined Patent Application No. 107521/1993 (Tokukaihei 5-107521), but also ferroelectric liquid crystal display apparatuses as disclosed in Japanese Unexamined Patent Application No. 20715/1991 (Tokukaihei 3-20715), plasma display apparatuses as disclosed in Japanese Unexamined Patent Application No. 43829/1994 (Tokukaihei 6-43829), etc.
In general, the matrix-type display apparatuses have such characteristics that a selection period is required independently for each scanning electrode, which makes it impossible to select a plurality of scanning electrodes at one time. In each of the described matrix-type display apparatuses, a display is performed by varying a voltage to be applied to the scanning electrode. Specifically, a selection voltage for determining a display state of a pixel is applied, and then a holding voltage for holding the selected display state of the pixel is applied. Lastly, an erase voltage is applied to erase the display state of the pixel. The display state of the pixel can be erased also by stopping the application of the holding voltage.
In the described display apparatuses, a gray shades display is enabled, for example, by the scanning method disclosed in Japanese Unexamined Patent Application No. 226178/1988 (Tokukaisho 63-226178). The scanning method will be explained in reference to FIG. 14.
FIG. 14 is a typical depiction of a scanning method for the matrix-type display apparatus including fifteen scanning electrodes L.sub.1 -L.sub.15, wherein the scanning electrodes L.sub.1 -L.sub.15 are selected in order according to the numbers 1-60 indicative of selection periods assigned to the top line. In respective blocks, bit numbers of data to be applied to pixels on the scanning electrodes L.sub.1 -L.sub.15 are assigned.
In this example, each bit of 4-bit data is applied to each pixel on the scanning electrode L.sub.i specified with an application of a selection voltage in each selection period. As a result, in each of the first through fourth selection periods, a pixel on the scanning electrode L.sub.15 displays the fourth bit, the pixel on the scanning electrode L.sub.1 displays the first bit, the pixel on the scanning electrode L.sub.3 displays the second bit, and the pixel on the scanning electrode L.sub.7 displays the third bit.
In FIG. 14, a non-selection voltage is applied to the scanning electrode L.sub.i to which the bit number is not assigned in each selection period.
As described, in the scanning method, a gray shades display is enabled by performing a scanning operation with a multiplex driving.
Here, a general arrangement of the ferroelectric liquid crystal display apparatus (hereinafter referred to as FLCD) to which the described scanning method is applied will be explained. The FLCD has a liquid crystal panel 61 as shown in FIG. 15. The liquid crystal panel 61 includes substrates 62 and 63 which face each other. The substrates 62 and 63 are made of a light-transmissive material such as glass, etc.
On the surface of the substrate 62, a plurality of transparent signal electrodes S made of, for example, indium tin oxide (hereinafter referred to as ITO) are placed in parallel. The signal electrodes S are coated with a transparent insulating film 64 made of, for example, silicone oxide (SiO.sub.2).
On the surface of the substrate 63, a plurality of transparent scanning electrodes L made of, for example, ITO, etc., are placed in parallel. These transparent scanning electrodes L are placed so as to cross the signal electrodes S at right angle. These scanning electrodes L are coated with a transparent insulating film 65 made of the same material as the insulating film 64.
On the insulating film 64 and 65, alignment films 66 and 67, to which uniaxial alignment process such as rubbing process, etc., is applied, are formed. For the alignment films 66 and 67, a polyvinyl alcohol, etc., is used.
The ferroelectric liquid crystals 68 are filled in a space formed between the glass substrates 62 and 63 which are laminated by a sealing agent 69 in such a manner that the alignment films 66 and 67 face each other, thereby forming a liquid crystal layer. The ferroelectric liquid crystals 68 are poured through an opening (not shown) formed in the sealing agent 69, and are sealed by closing the opening.
The substrates 62 and 63 are further sandwiched by two polarized plates 70 and 71 placed in such a manner that respective planes of polarization cross at right angle.
As shown in FIG. 16, the scanning electrode L (L.sub.o through L.sub.F) are connected to the scanning driver 81, and the signal electrodes S (S.sub.o through S.sub.F) are connected to the signal driver 82.
In the scanning driver 81, a one-bit scanning signal YI is transferred by the shift register 81a based on a clock CK, and is outputted from each output stage of the shift register 81a. The analog switch array 81b selects whether the selection voltage V.sub.c1 is to be applied to the scanning electrode L.sub.i, or the non-selection voltage V.sub.c0 is to be applied to the scanning electrode L.sub.k (k.noteq.i) depending on whether the signal outputted from the shift register 81a is in the High level or the Low level.
In the signal driver 82, the data signal XI is transferred by the shift register 82a based on the clock CK, and is outputted from each output stage of the shift register 82a. The output signal from the shift register 82a is held by a latch 82b in sync with a negative latch pulse LP. The analog switch array 82c selects whether an active voltage V.sub.S1 is to be applied to the signal electrode S.sub.j, or a non-active voltage V.sub.S0 is to be applied to the signal electrode S.sub.k (k.noteq.j) depending on whether the value held by the latch 82b is in the High level or the Low level.
In the FLCD having the described arrangement, a pixel is formed at a portion where the scanning electrode L and the signal electrode S cross. Then, by turning ON/OFF the lightening of each pixel, a display is performed on the entire liquid crystal panel 61.
As shown in FIG. 17(b), the liquid crystal molecules 91 in the pixel has a voluntary polarization P.sub.S in a direction perpendicular to the major axis direction. The liquid crystal molecules 91 move on the surface of a circular cone 92 having a vertical angle 2.theta. of twice as large as the tilt angle by receiving a force in proportion to a vector product of an electric field E generated by a potential difference between the application voltage to the scanning electrode L and the application voltage to the signal electrode S, and the voluntary polarization P.sub.S.
As shown in FIG. 17(a), when the liquid crystal molecules 91 are moved to an axis 93 by the electric field E, the liquid crystal molecules 91 become stable at position P.sub.1. When the liquid crystal molecules 91 are further moved to an axis 94 by the electric field E, and the liquid crystal molecules 91 become stable at position P.sub.2. Namely, the liquid crystal molecules 91 have the described two stable states.
Even if the liquid crystal molecules 91 are further moved by the electric field E, as long as the positions P.sub.1 and P.sub.2 do not vary, a restoring force is exerted onto the liquid crystal molecules 91 to move them back to the original stable state.
Here, by making a plane of polarization of either one of the polarization plates 70 and 71 shown in FIG. 15 to coincide with either one of the axes 93 and 94, two display states can be achieved. Specifically, the pixel having the liquid crystal molecules 91 in one stable state is in a bright display state, while the pixel having the liquid crystal molecules 91 in the other stable state is in the dark display state.
Not only the force generated by the electric field E but also a force in proportion to a product of a dielectric anisotropy .DELTA..epsilon. indicative of a difference in dielectric constant between the major axis direction and the minor axis direction of the molecule and the second power of the electric field E are exerted onto the liquid crystal molecules 91. Thus, the force exerted onto the liquid crystal molecules 91 is shown by the following formula: EQU F=K.sub.0 .times.P.sub.S .times.E+K.sub.1 .times..DELTA..epsilon..times.E.sup.2,
wherein K.sub.0 and K.sub.1 are constants.
For this reason, in the liquid crystal panel 61 in which an FLC material having a negative dielectric anisotropy .DELTA..epsilon. is sealed, when the electric field E is increased to a predetermined electric field E.sub.min, an increase in force by the negative dielectric anisotropy .DELTA..epsilon. becomes greater than an increase in force by the spontaneous polarization P.sub.S under an applied electric field E.sub.min, the force exerted onto the liquid crystal molecules 91 is maximized under the applied electric field E.sub.min. On the other hand, as a memory pulse width is known to be in reverse proportion to the force exerted onto the liquid crystal molecules 91, the memory pulse width is minimized under an applied electric field E.sub.min.
As the driving method for the FLCD utilizing the described characteristics, for example, JOERS/Alvey drive scheme (hereinafter referred to as a J/A drive scheme) is reported in "The JOERS/Alvey Ferroelectric Multiplexing Scheme" (Ferroelectrics, 1991, Vol. 122, pp.63-79) presented by Defense Research Agency in the FLC international conference (1991). The characteristics of voltage vs memory pulse width of the SCE 8 that is a FLC material available from BDH Ltd. described in the paper are shown in FIG. 18.
The circled data in FIG. 18 were measured while superimposing thereon a bias voltage of .+-.10 V shown in FIG. 19(a). On the other hand, in FIG. 18, the data marked "+" were measured while superimposing thereon a bias voltage of .+-.0 V shown in FIG. 19(b).
In the described driving method, the data in one screen is rewritten by scanning two fields. In the first field, as shown in FIG. 20(a), a voltage V.sub.SC is applied to the signal electrode S.sub.j when the selection voltage V.sub.CA is applied to the scanning electrode L.sub.i, thereby applying a voltage V.sub.A-C to the liquid crystal molecules 91 in the pixel at which the scanning electrode L.sub.i and the signal electrode S.sub.j cross each other. As a result, the liquid crystal molecules 91 can be switched to one stable state.
In the second field, as shown in FIG. 20(b), a voltage V.sub.SH is applied to the signal electrode S.sub.j when the selection voltage V.sub.CE is applied to the scanning electrode L.sub.i, thereby applying a voltage V.sub.E-H to the liquid crystal molecules 91 in the pixel at which the scanning electrode L.sub.i and the signal electrode S.sub.j cross each other. As a result, the liquid crystal molecules 91 are kept in the current stable state.
In the case of switching the stable state of the liquid crystal molecules 91 to the other stable state, first, in the first field, as shown in FIG. 20(a), a voltage V.sub.SG is applied to the signal electrode S.sub.j so as to applying the voltage V.sub.A-G to the liquid crystal molecule 91 in the pixel when the selection voltage V.sub.CA is applied to the scanning electrode L.sub.i. As a result, the stable state of the liquid crystal molecules 91 does not vary.
In the second field, as shown in FIG. 20(b), the voltage V.sub.SD is applied to the signal electrode S.sub.j when the selection volntage V.sub.CE is applied to the signal electrode S.sub.i, so as to apply the voltage V.sub.E-D to the liquid crystal molecule 91. As a result, the liquid crystal molecules 91 are switched from one stable state to the other stable state.
While the liquid crystal molecules 91 in other pixel are being switched from one stable state to the other stable state, the voltage is applied in the following manner.
As shown in FIG. 20(a), in the first field, the non-selection voltage V.sub.CB is applied to the scanning electrode L.sub.k (k.noteq.i) when the voltage V.sub.SC or the voltage V.sub.SG is applied to the signal electrode S.sub.j, thereby applying the voltage V.sub.B-C or the voltage V.sub.B-G to the liquid crystal molecules 91 in the pixel at which the scanning electrode L.sub.k and the signal electrode S.sub.j cross each other. As shown in FIG. 20(b), in the second field, the non-selection voltage V.sub.CF is applied to the scanning electrode L.sub.k when the voltage V.sub.SD or the voltage V.sub.SH is applied to the signal electrode S.sub.j, thereby applying the voltage V.sub.F-D or the voltage V.sub.F-H to the liquid crystal molecules 91 in the pixel at which the scanning electrode L.sub.k and the signal electrode S.sub.j cross each other. As a result, the stable state of the liquid crystal molecules 91 does not vary irrespectively of the applied voltage to the signal electrode S.sub.j.
The described driving method is applicable when the following conditions are satisfied:
Condition 1: Absolute values of the voltage levels -V.sub.s +Vd and V.sub.s -V.sub.d which respectively determine the voltages V.sub.A-C and V.sub.E-D shown in FIGS. 20(a) (b) indicate voltages of around 40 (V) in the characteristic diagram shown in FIG. 18 at which the force exerted onto the liquid crystal molecules 91 is in a vicinity of the maximum value; and
Condition 2: Absolute values of the voltage levels -V.sub.s -V.sub.d and V.sub.s +V.sub.d which respectively determine the voltages V.sub.A-G and V.sub.E-H shown in FIGS. 20(a) (b) are voltages of around 60 (V) in the characteristic diagram shown in FIG. 18 at which the force exerted onto the liquid crystal molecules 91 reduces from the maximum value.
Thus, the force exerted onto the liquid crystal molecules 91 with an application of voltage under the condition 1 becomes larger than the force exerted onto the liquid crystal molecule 91 with an application of voltage under the condition 2.
In order to apply the described driving method, the following conditions are also required:
The voltage V.sub.A-C takes two levels -V.sub.d and -V.sub.s +V.sub.d which are of the same polarity, and the voltage V.sub.E-D takes two voltage levels V.sub.d and V.sub.s -V.sub.d which are of the same polarity. On the other hand, the voltage V.sub.A-G takes two voltage levels V.sub.d and -V.sub.s -V.sub.d which are of opposite polarities, and the voltage V.sub.E-H takes two voltage levels -V.sub.d and V.sub.s +V.sub.d which are of opposite polarities. In the case of the same polarity, voltage levels -V.sub.S +V.sub.d and V.sub.s -V.sub.d which are easy to switch the stable state are selected. On the other hand, in the case of opposite polarities, voltage levels -V.sub.s -V.sub.d and V.sub.s +V.sub.d which are not easy to switch the stable state as compared to the case of the same polarity are selected.
The J/A drive scheme has been developed, for example, as a Malvern drive scheme that is disclosed in "A new set of high matrix addressing schemes for ferroelectric liquid crystal displays" (Liquid Crystals, Vo.13, No.4, 1993, 597-601). As shown in FIG. 21, in the J/A drive scheme (J/A in the figure), the selection voltage in the row voltage waveform is selected to have the same duration as a time slot T, while in the Malvern-2 and the Malvern-3 drive schemes respectively denoted by (M-2) and (M-3) in the figure, the selection voltages are selected to have durations of 2 times and 3 times of that of the time slot T respectively.
In the case of the FLCD as an example of the matrix-type liquid display apparatus, in the J/A drive scheme, drive voltages respectively having waveforms shown in FIGS. 20(a) and (b) are applied in the scanning of two fields required for rewriting the data of one screen, while in the drive scheme disclosed in "Color Digital Ferroelectric Liquid Crystal Displays For Laptop Applications" in SID '92, as shown in FIG. 22, by adopting an erase voltage (blanking pulse BP), the data in one screen is rewritten only in the second field.
However, in the described scanning method, the scanning electrode L.sub.i is selected in a discrete manner such as L.sub.15 .fwdarw.L.sub.1 .fwdarw.L.sub.3 .fwdarw.L.sub.7. In case of scanning with the existing driver IC, problems arise in that a complicated input signal (clock, data pulse, etc.) is required, more than necessary driver IC are required, etc. For this reason, in the drive circuit constituted by the existing driver IC, it is difficult to perform a multiplex gray shades display by the described scanning method.
The multiplex gray shades display is difficult to be performed with the existing driver IC also in the case where the selection voltage is held longer than the selection period like the Malvern-2 and the Malvern-3 drive schemes as shown in FIG. 21. Specifically, for example, when the selection voltage to be applied to the scanning electrode L.sub.1 is held longer than the selection period, the selection voltage of the scanning electrode L.sub.1 also affects the scanning electrode L.sub.3 to be scanned next. As a result, not only the selection voltage to be originally applied to the scanning electrode L.sub.3 but also the selection voltage being applied over the selection period for the scanning electrode L.sub.1 are applied to the scanning electrode L.sub.3.
Furthermore, the multiplex gray shades display is difficult to be performed also in the case of scanning shown in FIG. 22 in combination with an erase voltage. Specifically, in such scanning method both the selection voltage and the erase voltage are output ted in the same selection period (see FIG. 6); however, with the existing driver IC, it is difficult to output two signals simultaneously.